THE TEMPERATURE DEPENDENCE OF THE HYDROLYSIS OF METHYL ESTERS OF CERTAIN ALKYL-SUBSTITUTED BENZENESULPHONATES IN WATER1

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1 THE TEMPERATURE DEPENDENCE OF THE HYDROLYSIS OF METHYL ESTERS OF CERTAIN ALKYL-SUBSTITUTED BENZENESULPHONATES IN WATER1 ABSTRACT Rate data for the hydrolysis of a series of alkyl-substituted nlethyl benzenesulphonates are presented arid the resulting differences in derived parameters are compared and discussed in an analysis of substituent effects. Coincident with the hydrolysis studies to be reported on toluenesulphonates (I) a survey was made of the changes in rate and derived parameters for a series of allrylsubstituted benzenesulphonates with the aim of discovering the effect of systematic illcrease in the size of the anion being produced while the "mechanism" was lrept constant through use of the methyl ester. With tlie realization that the solvent reorganization accompanying the activation process did not necessarily include the total initial solvatioil shell of the ester (2), the more extensive study of tlie effect of such groups on solvatio~i was disco~itinued and tlie data already determined are published here. The importance of solvation on the rates of reactions in solution has long been recognized in principle (3) and the general trends analyzed in terms of charge developnlent or distribution. While far from optimum, the benzenesulphonic system provides a means of examining the effect of a substituent on the rate and thermodynamic parameters where there is no steric interference with solvation (para~substitution) and where the solvatio~is altered through the competition of more effective intramolecular interaction (ortho substitution). It is recognized that the latter is not ecl~~ivnlent to the simple screening of a charge by an alkyl group as considered by Everett and Wynne-Jones (4) in tliscussing the changes in solvation of anions with alkyl substitution b ~ is ~ more t closely allied to the effect considered by Evans (5) and Franl<lin (6) in calculating tlie heat of + solvation of tlie series Me -+ t-~fu carbonium ions, where the loss of solvation through solvent exclusio~~ was compensated for by internal resoliailce effects. TABLE I Effect of methyl substituents on relative rates of solvolysis in ethanol and water at 80' C Ethanol Water Acetic acid -- Ester klx1o'rel. rate klx103 Rel. rate klx106 Rel. rate - ~~ *~ef. 9. tref. 2. frates calculatetl from constants Table I, above, and Table V ref. 2. Ref. 11. 'Manuscript received November 4, Contribution front tlze Division of Pzire Chemistry, National Research Council, Ottawa, Canada. Isszied as N.R.C. No ?Sunzmer Studmt.

2 IIAMILTON AND ROBERTSON: HYDROLYSIS OF METHYL ESTERS 967 Whether an alkyl substituent group in a benzenesulphonate will be expected to interfere or in fact increase the solvation of the developing sulphonic anion in the activation process leading to hydrolysis will depend not only on its position and size, but also on the electronic nature of its effect, i.e., whether the effect is simply inductive, or shows evidence of hyperconjugation, or both. To evaluate the possible effect of hyperconjugation one must look for stabilizing influences arising from this cause both in the initial and in the transition states. Forinal presentation of typical resonance contributors to the stabilization of these two states are as follows: H 0 0 H6' H-C-9-S-06- i M OCH J H OCH, I~litial state -Transition state It will be obvious from the above that resonance cant-ributors will be more important in the initial state than in the transition state, since in the latter the developing negative charge will oppose any such electronic shift. Thus we anticipate the initial state for the para-methyl-substituted ester will be stabilized compared to the corresponding state for the unsubstitutecl ester, and the rate correspondingly reduced. This does not exclude the operation of an irtductive effect toward the same end, but evidence that a hyperconjugative effect is present may be gained from a comparison of the relative rates of solvolysis for a series of esters in ethanol and in water (Table I). If the effect of the methyl group were purely inductive the rate for tlie nieta methyl ester woulcl be the same or slower than that for the para methyl ester. Were the effect purely hyperconjugative, the substituent effect of the nieta methyl should be negligible. From Table I it is seen that the relative effect of the para methyl and ortho methyl are about additive to give the value for the 2,4-di~nethpl hydrolyzing in ethanol and water of 0.49 (calc.) in both cases and 0.52 and 0.53 (obs.) respectively. If the effect of the para methyl were con~pletely inductive, then the meta methyl should have approximately the same effect and we would anticipate the relative rate values for the 3,4-dimethyl to be 0.30 and 0.42 (calc.) compared with the observed values of 0.53 and 0.60 in ethanol and water, respectively. Hence it may be argued that some considerable fraction of the effect of the para methyl group must be attributed to hyperconjugative stabilization of the initial state. For tlie 2,4,6-trimethyl ester, it might be expected the second ortho methyl would not interact with the sulphonic group to the same extent as the lirst ortho methyl but woulcl have an effect somewhat like an additional para methyl group. Thus the relative rate for the 2,4,6-trimethyl ester might be expected to be intermediate between that calculatecl assuming the second ortho methyl to duplicate the first (calculated relative rate in etha~~ol 0.33, in water 0.27) and assuming it is a para methyl (calculated relative rate in ethanol 0.14 and in water 0.20). The observed values of 0.25 and 0.27 in ethanol and water respectively are in fair agreenlent with this analysis. The fact that the para methyl substituent may be changecl to other para alkyl sub-

3 068 CANADIAN JOURNAL OF CHEMISTRY. VOI, stituents with no change in relative rates (Table 11) compared with the unsubstituted ester may be taken as further evidence of compensation between the inductive and hyperconjugative effects. Taft (13) has explained the apparent constancy of the ionization constants of the 4-alkyl-substituted pyridines by a similar balancing of inductive and hyperconjugative effects and our results are in qualitative as well as quantitative agreement with his. TABLE I1 ERect of para alkyl groups on rate of hydrolysis of methyl benzenesulphonates in water at 60' C* Compound k X 10' sec-i k=/k~ Me-i)-t-BU 3.80f Me-p-?z- Pro 3.68f GS *Actual temperature 60.14' C. tref, 2. $Ref, 11. C'irhile comparisons of relative rates such as above give a valuable indication of trends, and largely provide the basis for our Itnowledge of the relati011 between structure and reaction rates, the fact that these ratios are temperature dependent encourages a ruore detailed exami~~ation in terms of the parameters derived from the temperature dependence of the rate. In 'Table 111 are given the rate data for three methyl esters over a range of 'TABLE 111 Kinetic data for the hydrolysis of three methyl-substituted benze~~es~~lphonic esters in water Me-3,4-dimethyl Me-2,4-dimethyl Me-2,4,6-trimethyl benzenesulphonate, benzenesulphonate, benzenesulphonate, Temp. klx105 sec-l kl x lo5 SCC+ k X lo5 sec-1 Standal-d deviation* [OR kob5- log kcalc *The average deviation from the mean of 3 or 4 separate kinetic runs is shown directly after the corresponding rate. The standard deviations between this observed value and the rate calculated from the derived equation [I] art. shown here. temperature. These data fit an empirical equation of the form within experimental error and values of the corresponding constants are sunlmarized

4 in Table IV. These constants may be interpreted as follows: d = - (A110*/2.303 R), B = (ACpc/R)+l, C = (ASo+-ACp*)/2.303 R + loglo klh, but it is well to recall that this interpretation filially rests 011 the assumption that an equilibrium exists between the initial and the trarlsitio~i state. Further, in our evaluation of AS+ we assume a frequency factor of log kt/h which is constant thro~lghout a series and in fact throughout all series. Since we normally are considering AAS* values within a give11 series such an assumption is probably acceptable at this preliminary stage of com- TABLE IV Summary of constants for eq~~ation [I] 'The thermodynamic parameters calculated from the above data are given in Table V. TABLE V Thermodynamic corlstallts at 25' C -- Benzenesulphonate Me-3,4-diMe Me-2,l-diMe RIIe-2,4,6-trihle k sec-i X AF' cal/mole 21,514 24,512 24,062 AHo* cal/mole 33,898 32,340 31,010 AH*?~s. IG cal/mole 21,855 21,257 21,455 - AC," cal/mole dcg AS* In Table VI are given the difference [unctions with u~isubstituted methyl benzenesulphonate as the standard. From these a much clearer picture call be gained of the effect of the substituent on the cletailed mechanism than was possible from a colisideration of relative rates. TABLE V1 EPiect of methyl substitution on thermodynamic constants for the hydrolysis of benzenesulphonic esters in water at 25O C -- Ester kl sec-'x los ~,AH',,I 6,AC,= 6,AS* We have noted elsewhere (1) that the p-ivie raises AH* by -200 cal while o-me lowers AH+ by cal. If we may assume, on the basis of the above evidence, that the solvation of the para alkyl group is not altered during the activation process, aside from the possibility of a small hyperconjugation effect (and the small AAS* supports this assumption), then the effect of the P-group is to increase the bond energy requiring

5 970 CANADIAN JOURNAL OF CHEMISTRY. VOL greater thermal activation to achieve the critical charge level characteristic of the transition state. The 3-Me group probably has a somewhat greater inductive effect (7), say AAH* f225 cal, while if the trend is maintained ortho methyl would be expected to raise AH* by cal (8). On this basis, the shift to be expected in AFI* for the 2,4-dimethyl will be (f ) = -300 compared to the observed value of -175 cal. To rationalize the value of AIlf for the 2,4,6-trimethyl ester we note from an examination of models that if a single ortho methyl interacts with the sulphonic group, the second ortho methyl will not be orientated for interaction. The second ortho methyl will then resemble a more effective ( cal) para methyl. The interaction of the first ortho methyl will cause a decrease in rotational entropy (&A* -2 e.u.) but this will not be duplicated by the second. Thus the predicted &AH* will be ( ) = -25 cal; observed +24 cal, and the &AS+ is of the same sign and magnitude as for a single ortho methyl. It will be noted that these results are in essential agreement with the additivity relations noted for the relative rates. Qualitatively, the differences in the heat capacity of activation are in accord with this analysis, but the crude state of our lrnowledge of the factors influencing this term limits interpretatioil of small differences. If the ACp* observed in hydrolysis arises from the temperature dependence of the AH* required to break the solvation shell, then it is probable that the observed differences in ACp* reflect differences in the degree of involvement of the solvent. As has been noted elsewhere, such large heat capacity differences call hardly be attributed to the solute alone. The changes in AC,+ are approximately additive. However, even considering the larger experimental error characterizing this parameter, the 2,4-dimethyl value for ACp* appears anomalous. EXPERI~VIEKTAL Solvents.-Ethanol (absolute) and water were as previously described (9, 2). To acetic acid (reagent) was added sufficient acetic anhydride so that after heating overnight at 100' C approximately 0.05% remained according to analysis of S. Brucksteiil (10). Esters.-The constants characterizing the sulphoilyl chloride and the esters are recorded in Table VII. There was no kinetic evidence (non-linear plots) of the presence of any competing impurity in the esters studied. TABLE VII Physical constants of methyl esters M.p. of Refractive Neutral Ester M.p. amide* index Density equivalent Me-p-Et , Me-p-n-Pro Me-p-isoPro Me-p-t-Bu Me-2,4-diMe Me-3,4-diMe Me-2,4,6-triMe *Of corresponding sulphonyl chlorides. These are in essential agreement with values found by previous workers and summarized by Suter (ref. 14). Kinetic studies.-the rates in water were determined by a conductance method. Details of the method, calculations, and assumptions have been discussed in detail in previous communications (11). The rate data in ethanol and in acetic acid followed the method described by Robertson (9) and Winstein (12) respectively.

6 HAMILTON AND ROBERTSON: HYDROLYSIS OF METHYL ESTERS 97 1 REFERENCES 1. RO~ERTSOX, R. E. Unpublished work. 2. ROBERTSON, I<. E. Carl. J. Chern. 35, 613 (1957). 3. INGOLD, C. I<. Struct~~re and mechanism of organic chemistry. Cornell University Press, Ithaca, N.Y p. 345 et seq. 4. EVERETT, D. H. and ~VYXNE-JONES, LV. F. I<. Trans. Faraday Soc. 35, 1380 (1939). 5. EVAKS,.A. G. Reactions of organic halides in solution. Manchester University Press FRAXI~LIX, J. L. Trans. Faraday Soc. 443 (1952). 7. TAFT, R. Steric effects in organic chemistry. John Wiley & Sons, Inc., New Yorl:, N.Y Chap. 13. Sec JAFFG, H. Chem. Revs. 53, 191 (1953). 9. ROBERTSON, R. E. Can. J. Chern. 31, 589 (1953). 10. BRUCKSTEIN, A. Anal. Chem. 28, 1920 (1956). 11. ROBERTSON, R. E. Can. J. Chem. 33, 1536 (1955). 12. WINSTEIN, S., GRUNWALD, E., and INGRAHAM, L. I. J. Am. Chenl. Soc. 70, 821 (1948). 13. TAFT, R. W. Private communication. 14. SUTER, C. M. The organic chemistry of sulphur. John Wiley & Sons, IIIC., London p. 458 et seq.

Enthalpy and entropy of activation for benzyl chloride solvolysis in various alcohol-water solvent mixtures

Enthalpy and entropy of activation for benzyl chloride solvolysis in various alcohol-water solvent mixtures H. S. GO LINK IN^ AND J. B. HYNE Departtnetlt of Clremistry, Vt2iversity of Calgcrry, Calgary,